Small-angle X-ray studies of soot inception and growth

The high spectral intensity of X-rays produced by the undulator at the Basi c Energy Sciences Synchrotron Radiation Center of Argonne's Advanced Photon Source has allowed us to perform small-angle X-ray scattering (SAXS) studi es of the initial distribution of soot particles formed by various fuels. S AXS provides an in situ probe of the morphology of soot in the region betwe en 1 and 100 nm and complements the ex situ technique of electron microscop y. The basic aspects of SAXS and its potential are illustrated with measure ment on a laminar flame of acetylene in air. The more complex fuel toluene has been studied in a flat-flame burner that supports a CH4/H-2/air or CO/H -2/air diffusion flame stabilized by N-2 co-flow. This burner produces a ne arly constant temperature region above the flame where the pyrolysis and co mbustion of the heavier fuels occurs. Kinetic information is obtained by pe rforming measurements of the scattered intensity profile as a function of t he height above the burner. These profiles have been reduced to give the me an radius and dispersion of a distribution of spherical particles. Mean rad ii between 0.8 and 18 nm have been observed. The smallest of these is a fac tor of ten smaller than previously detected with Lorentz-Mie scattering. Ne ar 1550 K, the soot distribution found in toluene shows a distinct step beh avior that is consistent with model calculations.